1. Field of the Invention
The present invention relates to an apparatus for detecting deterioration of an exhaust gas purifying catalyst, by processing output signals from oxygen sensors installed respectively upstream and downstream of the catalyst.
2. Description of Related Art
In conventional exhaust gas purifying systems provided with oxygen sensors installed respectively upstream and downstream of a catalyst, the period of the output signal from the downstream oxygen sensor is longer during normal operation, than that of the upstream oxygen sensor, due to the catalyst storage effect. However when this storage effect declines due to deterioration of the catalyst, the period of the downstream oxygen sensor output signal shortens to approximately the same period as that of the upstream oxygen sensor. Upon catalyst deterioration, the amplitude of the downstream sensor output signal becomes larger compared to that when the catalyst is operating normally.
An apparatus is known to utilize this characteristic for detecting deterioration of the catalyst, as disclosed in Japanese Unexamined Patent Publication 61-286550 (U.S. Pat. No. 4,739,614), the ratio of the output signal periods of the upstream and downstream oxygen sensors is obtained, and when the ratio of their periods is smaller than a specified value or when the amplitude of the downstream oxygen sensor output signal exceeds a reference value, deterioration of the catalyst is determined to have occurred.
However, in this apparatus, when the output characteristic of the upstream oxygen sensor deteriorates and its response is delayed, the downstream oxygen sensor output signal swings between rich and lean sides at the same period since the output period of the upstream sensor output signal becomes lengthened. As a result, the ratio of output signal periods between the upstream and downstream oxygen sensors decreases and the catalyst is likely to be mistakenly determined to have deteriorated even though its operation is correct.
The air/fuel ratio feedback correction coefficient FAF, which compensates the air/fuel ratio of the mixture so that a nearly stoichiometric air/fuel mixture is supplied to the engine, alternates in steps to the rich and lean sides as shown in FIGS. 13A-13C, each time the output signal from the upstream oxygen sensor reaches a reference voltage VR1 which indicates the stoichiometric air/fuel ratio. However as described above, when the upstream oxygen sensor deteriorates, the output signal period lengthens as shown in FIGS. 14A-14C and in response, the amplitude of the air/fuel ratio feedback correction coefficient FAF increases from W1 to W2 and fluctuations in the air/fuel ratio become larger. These larger fluctuations also cause the amplitude of the downstream oxygen sensor output signal to increase. Therefore, if the catalyst deterioration is determined to have occurred in the same way as in the prior art when the large amplitude of the downstream oxygen sensor occurs, the catalyst deterioration is mistakenly determined to have occurred due to upstream oxygen sensor deterioration.
To deal with these drawbacks, the invertors have conceived an apparatus which measures the response delay time of the downstream oxygen sensor and determines catalyst deterioration has occurred when the response delay time has become shorter than a reference value. The response delay time, here, is the time from inversion between rich and lean sides of the air/fuel ratio feedback correction coefficient FAF until the downstream sensor oxygen output signal reaches the level of the reference voltage VR2.
However the inversion timing for the air/fuel ratio feedback correction coefficient FAF being the timing for response delay measurement of the downstream oxygen sensor, varies according to the response of the upstream oxygen sensor. Since the response of the upstream oxygen sensor fluctuates according to the extent of deterioration and variations in sensor quality, simply measuring only the response delay time of the downstream sensor still allows the upstream sensor response to affect by such variations and the detection accuracy of deterioration of the catalyst will decline. The inversion timing of the air/fuel ratio feedback correction coefficient FAF will also fluctuate according to variations in engine operating conditions thus affecting the response, delay time.
In order to minimize the influences from variations caused by the engine operating conditions, increasing the measurement repetitions of the downstream oxygen sensor response delay time and comparing the average value thereof with a specified value is conceivable. However in this case also there would be no improvement in effects received from upstream oxygen sensor response variations. Increasing measurements of response delay time adds to the data processing load and, hence, processing capacity for other purposes is limited or a large increase in processing capacity is required.